Process for manufacturing a non-woven fabric by hydrodynamic needling, and product of said manufacturing process

ABSTRACT

It is known to make a nonwoven from polyvinyl alcohol fibers. The special advantage of these PVA fibers is their ability to dissolve in water. The fleece is therefore mechanically needled for compaction. The invention describes a method that allows hydrodynamic needling as well. Special parameters for further treatment during water needling and during subsequent drying characterize the additional features of the invention.

BACKGROUND OF THE INVENTION

In the journal ITB Nonwovens, Industrial Textiles, 4/95, pages 20-25,"Degradables or the Recycling Economy for Disposables," or in thejournal Chemical Fibers International, Vol. 46, April 1996, page 102, "ANew Water-Soluble PVA Fiber for Nonwovens Application," a specialchemical fiber is described made from a hydrophilic synthetic polymer.In addition to other important properties, it has the special ability todissolve in water at certain temperatures without leaving a residue. Thefiber consists of a polyvinyl alcohol and can be processed by weaving orby nonwoven technology to form a sheet material from which any desiredarticle of clothing or the like can be produced. These sheets made ofPVA fibers however are used especially in hygienic products becauseafter being used during surgery in a hospital for example, they can bedisposed of easily and rapidly if contaminated with blood and the likeby dissolving them in hot water.

It is known that these PVA fibers can be processed by means of a card oraerodynamically to form a nonwoven. To lend them sufficient strength,these fleeces are compacted by mechanical needling technology and thenprocessed to produce the desired articles of clothing. It has been foundthat fleeces with these fibers that have been compacted by mechanicalneedling technology can be produced at rates that are too low forindustry. In addition, when mechanical needling is used, there is a riskof holes being formed, which is disadvantageous for the desiredimpermeability of the hygienic articles to water.

The goal of the invention is to develop a method by which the nonwovenmanufactured as usual from these PVA fibers can be compactedcontinuously at a higher rate than by the mechanical method, and thus aproduct can be produced that is uniformly compacted and is essentiallyimpermeable to fluid.

DISCLOSURE OF THE INVENTION

It is known to use hydrodynamic needling alone to produce a compactedendless fleece product. The fleece web coming from a fleece-layingmachine such as a card or together with a cross-layer is subjectedimmediately thereafter to needling water jets to compact the fleeceproduct. Then the wet fleece must be dried. The idea of the invention isto process a nonwoven by this method, said nonwoven consisting at leastpartially of PVA fibers. Initially it would appear impossible to usethis water needling method alone as a compaction method for such achemical fiber, since the fibers come in contact with water for aprolonged period of time during needling and therefore there is a riskof their dissolving. This danger exists, if not during needling itself,then at least during the drying of the wet fleece product immediatelyafterward, since drying is not possible without heat.

It has now been found according to the invention that it is neverthelesspossible with several special parameters to compact a fleece productmade of these fibers using the hydrodynamic method. Thus for example itis advantageous if the previously moistened nonwoven is subjected onceon both sides to the water jets and by several water jets in sequence ineach case with the energy of the jets of the next nozzle beam alwaysbeing higher, for example from 50 bar to 120 bar on the first side andfrom 120 bar to 160 bar on the second side. The last needling on eachside should be performed at about 80 bar and performed with a largernumber of water jets distributed across the width of the nonwoven inorder to produce a uniformly smooth surface. Under these conditions, afleece weight of 40 to 150 g/m² can be compacted. The transport speed ofthe nonwoven during compaction is 70 m/min or more. Thefleece-processing rate depends only on the possible fleece-laying rate.The production rate is adjusted to the respective fleece weight, but itis always lower at higher weights.

It is important how the drying parameters are defined. Initially theneedled fleece must be dewatered mechanically before it is dried, bysqueezing or by suction for example, in order to achieve a level ofmoisture that is not more than 100%. Then the fleece must be dried bydrying air which is not heated to a temperature greater than 120° C. Itis especially advantageous for the drying and the ventilation to beperformed at the same time, in this case on a rotating screen drum withinternal suction, and to increase the air speed in the fleece by a highfan rpm, up to 4 m/second. Various tests have shown that with thismethod there is no damage to the PVA fibers. Both during drying and alsoduring the hot final processing that follows, there were no visibledisadvantages like the brown spots that usually occur otherwise.Basically, drying is also possible using a belt dryer, with ventilationalso being produced, or with an IR dryer, etc.

If the fleece is also to be given impregnation such as foam or liquidimpregnation that makes it water-repellent, it is advantageous toperform this step after a first drying down to 30% moisture contentunless impregnation takes place wet-in-wet. Then after the first drying,the second drying stage should be performed exactly like the first at atemperature of up to 120° C. and the fleece dried completely. It is alsopossible to perform impregnation only after drying for example down to5% moisture content. Following complete drying of the needled fleece, itis no problem to crosslink the fleece at temperatures up to 210° C.

It is known that a fleece made of these PVA fibers can be provided withan additional layer of pulp or paper in order to increase thewater-repellent property of the nonwoven. Foam impregnation, liquidimpregnation, and also advantageously in the method according to theinvention, application of a layer of this kind in pulp form or as tissuepaper, can be used, and then bonding the layer to the needled fleece,with said layer being laid down on the fleece prior to the secondneedling for needling on the back side simultaneously with the fleece,said layer being bonded with the fleece during the needling that thentakes place.

The method according to the invention produces a novel product. Thesubject of the application also extends to a fleece product made of PVAfibers that is compacted by water needling on both sides for example andfinally is dried as well.

We claim:
 1. Method for producing an endless fleece product, comprisinglaying down a fleece web by a fleece-laying machine, said fleece webcomprising PVA (polyvinyl alcohol) fibers; striking the surface of thefleece with needling water jets to compact the fleece; mechanicallypartially dewatering the needled fleece; and then drying the fleece withair passing through the fleece, the air passing through the fleecehaving a through-flow speed of more than 2 to 4 m/sec and a temperatureup to 120° C.
 2. Method according to claim 1, further comprising, aftermechanically partially dewatering the water-needled fleece, subjectingthe fleece to a second drying stage comprising passing air through thefleece at a maximum temperature of 120° C.
 3. Method according to claim1, further comprising briefly heating the dry fleece to a temperature ofabout 210° C. for curing.
 4. Method according to claim 1, furthercomprising impregnating the fleece with a finishing agent that makesfibers of the fleece water-repellent.
 5. Method according to claim 1,characterized in that the step of striking the surface of the fleecewith needling water jets comprises striking the fleece with needlingwater jets on both sides of the fleece.
 6. Method according to claim 1,characterized in that the fleece composed of PVA fibers is laid downduring water-needling on a belt supporting the fleece with a freesurface of 18 to 25% and/or 80 to 120 mesh (wires per inch) with singleor multiple binding.
 7. Method according to claim 1, characterized inthat the needling of the fleece is performed by several nozzle beams insuccession with slightly increasing water pressure each time.
 8. Methodaccording to claim 5, characterized in that the step of striking thesurface of the fleece with needling water jets comprises striking afirst side of the fleece with several nozzle beams in succession withslightly increasing water pressure each time and then striking a secondside of the fleece with several nozzle beams in succession and at waterpressures higher than those used on the first side and that increaseslightly.
 9. Method according to claim 8, further comprising striking atleast one side of the fleece with needling water jets from an additionalnozzle beam charged with reduced water pressure and a higher holedensity than the previous nozzle beams.
 10. Method according to claim 1,further comprising coating one side of the fleece with pulp or paperweighing about 15 g/m² and conducting a second needling for joining thiscoating with the fleece that has already been needled on one side. 11.Method according to claim 3, characterized in that the step of brieflyheating the dry fleece to a temperature of about 210° for curing iscarried out from 20-30 seconds.
 12. Method according to claim 7,characterized in that the needling of the fleece is performed by nozzlebeams at slightly increasing water pressures of about 50, 80 and about120 bars.
 13. Method according to claim 8, characterized in that theneedling of the first side of the fleece is performed by nozzle beams atslightly increasing water pressures of 50, 80 and about 120 bars, andthe needling of the second side of the fleece is performed by nozzlebeams at slightly increasing water pressures of 120, 140, and 160 bars.14. Method according to claim 9, characterized in that the reduced waterpressure is about 80 bars.
 15. Fleece product produced by the methodaccording to claim
 1. 16. Fleece product formed at least partially ofPVA fibers compacted by water needling on both sides and then dried withair passing through the fleece.